identifying the need of JSF over JSTL/EL

I see this question in Mark Cade and Humphrey Sheil's book- chapter 3 :-

You have been asked to evaluate multiple web presentation technologies for ABC Corp.Their priorities are future-proofing, tooling support from IDEs and the ability to render multiple versions of the same component for different devices. What do you recommend toABC?
A)Use JSF components as part of a Servlet.
B)Use JSTL and the EL as part of JSP pages
C)Use JSF components as part of JSP pages.
D)Use JSTL and the EL as part of Servlets.

WHy is answer C correct and is better than B?The way answer is explained in the book ,it seems future-proofing and tooling support cannot be achieved by JSTL/EL?Any idea why?